1. Field of the Invention
This invention pertains to an improved method of securing a plastic or glass lens as a light fixture window lens, where the window lens does not have to be opened for normal maintenance, and to the resulting improved lighting fixture.
2. Description of the Prior Art
Commercial lighting fixtures each enclosing one or more bulbs or lamps usually include a large window opening through which light emanates from the bulbs or lamps. The normal material used for closing this window is clear glass. In some installations frosted or otherwise partially opaque glass is employed to soften or diffuse the light.
In a typical installation of a high intensity discharge lamp, such as a mercury vapor lamp, the window opening closed by a pane of clear glass may be approximately two feet by two feet. The entire lighting fixture may be one of a plurality mounted in a high ceiling over a laboratory, industrial process area, a warehouse of similar area.
Although plastics have been substituted for glass in many applications, such as for canopies of airplanes, contact lenses, lenses for photocells and many, many other applications, previous to the development of the lens described in U.S. Pat. No. 3,812,342, substitution had never been totally satisfactory in the high temperature, prolonged use, often abusive environment that high intensity discharge lamps are subjected to. An article by J. T. Barnes appearing in Lighting Design & Application, December 1972, is believed to reflect the state of knowledge as of its publication date. The findings of Barnes is that for short-term use in a high temperature, high ultraviolet environment, there are some coated polycarbonates that might be considered reservedly acceptable. For general low temperature use, some acrylics (such as Plexiglas) are acceptable. For high temperature use (over 105.degree. C.), there was no known substitute for glass. The sole exception was that in extreme breakage areas or hazardous locations with inside temperatures not in excess of 125.degree. C., polycarbonate may be substituted, provided a very limited service life is acceptable. There are so many characteristics besides accommodation to the above that have to come together in a single plastic, that it was a remarkable discovery that any plastic could be suitable. For example, for a clear light fixture lens application, there had to be good optical, low-backscatter properties, not just initially, but after months and even years of use. Since most plastics, and even some fluoroplastics, degrade when exposed to ultraviolet light, the selection of a plastic with acceptable optical properties was extremely difficult. Further, as noted above, the high temperatures attendant to high intensity discharge lamps is also a critical problem pertaining to the selection of a suitable material.
Moreover, unless there was a vast saving in weight, the economics of the substitution did not make any sense. The most common glass substitute for large panes is probably Plexiglas, which is rigid and commonly seven-thirty-seconds inch thick (approximately 5.56 mm.). Because it optically degrades under high temperature conditions and for other reasons, it is not acceptable. Lexan, another sometime glass substitute in other contexts, does withstand high temperatures better than Plexiglas, but it yellows to an objectionable extent.
The primary advantages of glass as a fixture closure or lens include its low cost, its ready availability, its resistance to high operating temperatures, such as emanate from high wattage lamps, its resistance to change in color and opaqueness, even over a prolonged period of time, and its uniform light transmittance qualities over the full range of the visible spectrum.
Glass as a closure for such a fixture has a number of disadvantages, however. First, it is breakable. Should something accidentally strike the glass or should the bulb within the fixture explode, the glass is very likely to break causing not only an inconvenience to the persons working in the area, but also creating a hazard. So-called non-breakable tempered or safety glass is available and is used. But, even tempered glass does break. In such case, beads result, rather than jagged pieces, but even beads can be hazardous.
Further, glass is thought of as being relatively slick and therefore resistant to the build-up of dust. However, as most persons can attest to who have had experience with dust build-up on glass, it really does not take very long for an appreciable amount to accumulate. The rapid accumulation of dust results in reduced illumination from the fixture and a requirement to clean the fixture. The more often someone has to clean the fixture, the more expensive is the maintenance.
The glass used in a fixture such as the one described above is also an appreciable percentage of the overall weight of the fixture. A lighter window closure would effect a reduction in manufacturing and shipping costs.
As previously mentioned, U.S. Pat. No. 3,812,342 reveals an opening or lens structure which is suitable for many types of lighting installations. However, one type of installation which is not covered by the structure there revealed is for a fixture which is entered from the rear for bulb replacement purposes. Such a fixture is prevalent in outdoor sporting installations (e.g., for lighting tennis courts, baseball diamonds, and the like) and is known as a sports light. For purposes hereof, any light fixture which is not entered through the window lens, such as in the manner illustrated in the '342 patent, is referred to as a sports light, regardless of the actual use or installation of such light.
Fluoroplastics, or fluorocarbon polymers, all have the property of having a resistance to high temperatures, being light weight in small thickness and being unbreakable. Some fluoroplastics, and in particular Teflon FEP (a fluorocarbon copolymer made by polymerizing a mixture of tetrafluoroethylene and hexafluoropropylene), have the additional properties of being nearly transparent in thin-film form (no thicker than about 10 mm.), having a high and uniform light-spectrum transmittance, having a long-term aging quality without appreciable discoloring, and having an extremely low coefficient of friction and therefore providing a dust resistant surface. It has been discovered that a lighting fixture closure made of such a material has sufficiently equal or superior qualities in all of its necessary characteristics that it is an overall superior closure to that of glass.
In a lighting fixture which is entered from the rear for bulb replacement, the previous method of securing a fluorocarbon lens to said fixture as described in U.S. Pat. No. 4,120,023, was to stretch a lighting fixture closure in the form of a thin-film transparent lens or window for the fixture over the rim and to squeeze such lens into place preferably using a gasket and a metallic steel band. When the housing of the fixture is aluminum, heat expansion of the fixture during the time the internal light is lit actually tightens the band in place due to the difference between the temperature coefficient of aluminum compared with that of steel.
The manner of securing the fluorocarbon lens on the lighting fixture illustrated in U.S. Pat. No. 4,120,023 has several disadvantages. First, it is difficult to install in that the gasket must be forced over the extended portion of the lens. Second, field reliability of the fixture has been inadequate in that occasional water leakage has been encountered when the lighting system is allowed to cool, since the heat expansion, as previously mentioned, that was utilized to tighten the band is not present when the lighting fixture is not in operation. Third, the cost of utilizing the system described in the '023 patent is high, in that the fluorocarbon lens must extend well past the end of the housing so that it can be totally covered by the gasket rim along with the cost for the difficult job of forcing the closing band back over the rim, gasket and lens.
Although it has been recognized that Teflon FEP could be bonded to adhesives, it has been pure speculation that either Teflon FEP or Teflon TFE (a fluorocarbon homopolymer called polytetrafluoroethylene) or other suitable thin plastic films could actually be suitably secured to a light fixture frame via adhesives. For example, it was not known if such adhesives would so weaken such films so as to cause tearing or to age such films to cause cracking after short use or to cause hardening so that the adhesives would pull loose or that some other problem would be encountered.
It is therefore a feature of this invention to provide an improved method of securing a plastic lens to a sports type of lighting fixture which does not require front entry for maintenance of the light source.
It is another feature of this invention to provide an improved method of securing a plastic lens especially a fluorocarbon lens that extends past the edge of the fixture to an extending rim from a sports type lighting fixture that does not require front entry for maintenance of the light source.
It is a further object of this invention to provide a simple, inexpensive method of securing a plastic lens to sports type lighting fixtures with non-front entry access to the light source, so as to provide greater security for the internal elements of the fixture from external weather conditions.